Conventionally, there are known electron tubes that are used in detecting whether or not there is a flame based on ultraviolet radiation that is emitted from a flame in a combustion furnace or the like. This electron tube is provided with a closing device that is filled with a specific gas and sealed, an electrode support pin that passes through the closing device, and two electrodes supported, parallel to each other, within the closing device by the electrode supporting pin. In this electron tube, when one of the electrodes that is disposed facing a flame is illuminated with ultraviolet radiation in a state wherein a specific voltage is applied between the electrodes through the electrode supporting pin, electrons are emitted from that electrode through the photoelectric effect, and the electrons are stimulated one after another to cause and electron avalanche to the other electrode. Because of this, it is possible to detect the presence of a flame through a change in the impedance between the electrodes, a change in the voltage between the electrodes, measuring the current that flows between the electrodes, or the like.
In such an electrode tube it is necessary to increase the amount of luminescent flux or the density of the incident ultraviolet radiation in order to increase the sensitivity. While simply increasing the diameter of the electron tube would increase the incident ultraviolet radiation, if such a large electron tube were equipped in a combustion furnace device, there would be the risk that this could cause too great a change in the shape or the volume of the combustion furnace, which could have a deleterious effect on the furnace reaction, or the like. Given this, conventionally a lens has been placed between the light source (the flame) and the electron tube, to focus the ultraviolet radiation that is incident on the lens, to achieve an improvement in the amount of flux or the concentration of the ultraviolet radiation that is incident into the electron tube.
However, the materials for lenses that are transparent to ultraviolet radiation (quartz glass, or the like) are expensive, and thus mass production scale-up and cost reductions have been difficult. Given this, in recent years there have been proposals for reflecting mirrors wherein the reflecting surface is formed from a parabolic surface instead of using a lens. See, for example, Japanese Unexamined Patent Application Publication 2011-214893.
However, when the reflecting surface of the reflecting mirror uses a simple parabolic surface, the cross-sectional shape of the focused ultraviolet radiation will be either a point or a hollow circle, so it has been difficult to focus uniformly as a plane. Given this, if, for example, focusing the ultraviolet radiation onto a single point, electrons will be emitted locally from that single point, which will cause the location of emission of the electrons to break down, resulting in a loss of sensitivity in detecting the flame using the electron tube. Because of this, a reflecting member that is able to illuminate a given region uniformly with reflected light wherein electromagnetic radiation, such as ultraviolet radiation, is reflected is desirable.
Given this, an aspect of the present invention is to provide a reflecting member and a flame sensor able to cause reflected electromagnetic radiation to illuminate a given region uniformly.